1. Field of the Invention
The present invention relates to a scorotron type charging apparatus for charging the surface of the photoconductive body of, for example, an electrophotographic copying machine at a fixed potential and, more specifically, to a voltage controller for controlling the voltage of a grid electrode.
2. Description of the Related Art
As shown in FIG. 3, a scorotron type charging apparatus employed in an electrophotographic copying machine, in general, comprises a shielding case 1 formed of aluminum or the like and having an open lower end, a corona discharge electrode 2, such as a tungsten wire or the like, extended within the shielding case 1, a counter electrode 3 disposed opposite to the opening of the shielding case 1, a grid electrode 5 disposed between the shielding case 1 and the counter electrode 3, and a dc power source 4 for producing a corona. A bias voltage is applied to the grid electrode 5 to control the electrostatic potential of the surface of a photoconductive body 6, i.e., a charged body, placed between the corona discharge electrode 2 and the counter electrode 3. The scorotron type charging apparatus, as compared with a corotron type charging apparatus (not shown) which is not provided with any grid electrode, features the capability of uniformly charging the surface of a photoconductive body.
The bias voltage applied to the grid electrode 5 must be controlled to enhance the uniformity of the distribution of the electrostatic potential over the surface of the photoconductive body 6. The conventional scorotron type charging apparatus employs, as shown in FIG. 3, a parallel circuit consisting of a series constant-voltage dc power supply 7, i.e., a dc power supply having a control circuit connected in series to the load for voltage regulation, and a load resistor 8, connected in series between the grid electrode 5 and the counter electrode 3 to control the bias voltage. Another conventional scorotron type charging apparatus, as shown in FIG. 4, employs a parallel constant-voltage dc power supply 17, i.e., a dc power supply having a control circuit connected in parallel to the load for voltage regulation, connected to a grid electrode 5 and a counter electrode 3. A third conventional scorotron type charging apparatus, as shown in FIG. 5, employs a constant-voltage passive circuit 27 connected to a grid electrode 5 and a counter electrode 3. A constant-voltage passive device 27a as shown in FIG. 6(a) or a series circuit of a constant-voltage passive device 27a and a resistor 28 as shown in FIG. 6(b) is used as the constant-voltage passive circuit 27.
The conventional controller including the parallel circuit of the series constant-voltage dc power supply 7 and the load resistor 8 as shown in FIG. 3, however, consumes electricity uselessly by the load resistor 8, and is unable to change the voltage of the grid electrode 5 easily because the resistance of the load resistor 8 must be varied to vary the voltage of the grid electrode 5 over a wide range.
The controller employing the parallel constant-voltage dc power supply 17 as shown in FIG. 4 also has problems. The controller requires surplus power because the power provided through the control circuit by an unstable dc power supply (not shown, but included in the dc power supply 17) is very low, namely, the output conversion efficiency of the dc power supply 17 is very low, and the output efficiency of the parallel constant-voltage dc power supply 17 drops greatly when the voltage of the grid electrode 5 is varied over a wide range.
The controller employing the constant-voltage passive circuit 27 as shown in FIG. 5 is unable to control the voltage of the grid electrode 5 optionally with high accuracy because a voltage is applied to the grid electrode 5 in combination with the constant-voltage passive devices 27a, and through a selector switch 29 as shown in FIG. 7(a) or 7(b).